1. Field of the Invention
This invention relates to an optical system for a semiconductor laser beam, particularly to such a system equipped with a semiconductor laser. More particularly, the present invention relates to an optical system for a semiconductor laser beam which enables the laser beam to be converged to a spot of small diameter over a broad output range of the semiconductor laser.
2. Description of the Prior Art
Semiconductor lasers fabricated using semiconductor chips are used for generation of scanning beams in different kinds of equipment such as optical scanners for reading and writing of information. The semiconductor laser has numerous advantages over other types of lasers, including the gas laser. Aside from being more compact and less expensive, it also consumes less power and its output can be modulated by control of the driving current supplied thereto, in what is referred to as direct analogue modulation. The semiconductor laser is especially convenient for use in an optical scanning device for writing information since it allows the aforesaid direct modulation to be conducted using a signal generated in accordance with the image information to be recorded.
In this connection, however, it is known that the light produced by a semiconductor laser includes both stimulated emission and spontaneous emission. The relation between the amounts of these two kinds of light and the amount of current applied to the semiconductor laser will be explained with reference to FIG. 4.
In the graph, the line a represents the relation between the driving current and the spontaneous emission output, while the line b represents that between the driving current and the stimulated emission output. As will be noted, no stimulated emission is produced until the driving current reaches a threshold current value Io, so that up to this current value, only spontaneous emission is generated. While the amount of spontaneous emission output increases gradually with increasing driving current, once the driving current has gone beyond the threshold value Io and the amount of stimulated emission output becomes large, the spontaneous emission comes to account for such a small percentage of the total light output that for all intents and purposes the output can be considered to consist solely of stimulated emission. In the graph, the line c shows the relationship between the amount of driving current and the total light output (i.e. the combined spontaneous emission and stimulated emission output) of the semiconductor laser.
On the other hand, as disclosed for example in U.S. Pat. Nos. 4,258,264, 4,276,473, 4,315,318 and 4,387,428 and Japanese Unexamined Patent Publication No. 56(1981)-11395, the applicant previously proposed a radiation image recording and reproduction system in which recording, read-out and reproduction of radiation images are conducted using a stimulable phosphor sheet as the recording medium. In this system, since the images to be recorded and reproduced have a wide density range, it is necessary in recording the images on the recording medium to modulate the recording beam over a broad dynamic range of 1 : 100-1000. Thus when the light source used for producing the recording beam in the radiation image recording and reproduction system is a semiconductor laser that is directly analogue modulated, it becomes necessary to use the semiconductor laser even in the low output region where the influence from the spontaneous emission produced is large. Differently from the stimulated emission, however, the spontaneous emission includes a mixture of various angular components. Moreover, as in the case of a vertical multimode semiconductor laser, for example, the spontaneous emission consists of spectral components ranging over about 40 nm, in contrast to the spectral range of approximately 2 nm for the stimulated emission. As a result, it is impossible using a convergent lens to converge the spontaneous emission to a spot of such small diameter as is possible in the case of the stimulated emission. Therefore, when the semiconductor laser is used both in the high output region where the stimulated emission dominates and in the low output region where the spontaneous emission dominates, there arises a problem of degraded spatial resolution in the scanning operation.
Further, the semiconductor laser beam source equipped with a semiconductor laser is generally provided with a collimator lens, convergent lens or the like. Thus when, for example, there is used a multimode semiconductor laser which produces spontaneous emission having a spectral range extending over 40 nm, as against a 2 nm range for the stimulated emission component, the spontaneous emission is apt to be effected by the chromatic aberration of these lenses, making it difficult to converge the light to a spot of small diameter without employing expensive lenses that have been precision compensated for chromatic aberration.